36v or 72v first build advice

[MarketyMark]

I have a lot of 36v battery packs (Dewalt DCB407). They have built in bms and are weather resistant. I used them to build an electric kayak and a modular power pack for power outages and camping. I'm going to release a series of youtube videos, using the same packs to power multiple devices and want to complete an ebike build next. I have good electronics knowledge, access to a 3d printer, and metal cutting and welding gear. I tend to "go big or go home" when it comes to projects like this.

With my existing battery packs, I can either stick with the native 36V or put them in series and go with 72V. I have a Rocky Mountain Fusion 10 bike with a standard 135mm dropout width. It looks to me like 72v hub motors don't really come in 135mm widths and I'm not sure bending the aluminium frame is a good idea. I think my ideal motor is a 48v or 52v 1500W. 500W max with 36v seems a bit anemic.

Is it possible to use a 72v inverter and dial it down to 52v and use a 52v motor? Or is that not how it works?
Are there 72v hub motors that will fit a 135mm dropout?

Thanks for the feedback. I have a thick skin so give it to me straight!

 

[amberwolf]

Motors dont' really have a "voltage"; just are typically sold as running at some stated max speed at some stated voltage.

So you pick a motor that fits the bike and goes as fast as you need it to, and can handle the power needed to do the job you need the bike to do for you. Then pick a controller that can feed taht much power to it, at whatever battery voltage you need to use it at. Then build your battery setup to be capable of supplying the power the system will need to do the job you need it to do. Then just don't use full throttle (so it won't go too fast), or if you have a "lead thumb" just limit the speed in the controller settings, if it's programmable.

the Grin Tech motor simulator at ebikes.ca is useful for figuring out what motor and controller and battery are needed to do a particular job under various conditions.

 

[dogman dan]

Also, 36v does not limit you to 500w. The typical "bike" hub motors are rated at 500w. The ones with internal planetary reduction gears tend to do best if kept below 1500w, with the typical off the shelf 36v kit set at around 750 watts, with the controller doing the limiting. That's one of the things a controller controls, is max wattage. But you can get stronger controllers, and run more, even at 36v. Typically, a 36v set up goes around 20 mph, which is actually moving at a good clip for typical bike riding on less mountainous terrain. 48v gets you to or near to 30 mph, and 72v 40 mph. More wattage at 36v will tend to be a little bit faster, but a lot faster to get to that max speed, and more power for a hill.

The direct drive type 500w rated motor can take more wattage. For short durations, like 30-40 min, they can handle 3000w. Thats 72v at 40 amps. Or 80 amps at 36v. I personally found 48v 40 amps a real nice choice. 2000w of power makes it accelerate more like a motorcycle, but 40 mph on streets through the road debris, potholes, manhole cover humps, etc too stressful, and actually dangerous.

As you see though, your choice of battery is not good for 48v. I would start with a stronger 36v controller. 36v and 40 amps still rides real nice, even if the top speed is low compared to 48v. You will have to parallel a lot of batteries to get 40 amps from them without damaging them. But that is no problem. Series connecting a lot of bms equipped packs is not as good. The packs are just not designed to be series connected. There are though, 48v mower batteries.